The present application is based on Japanese Patent Applications No. 2001-226200 and 2001-393317, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to an actuator having an inductance, and, more particularly, to a drive circuit for an actuator having an inductance, such as a brushless motor, a DC motor, a voice coil motor, or an electromagnetic actuator.
2. Related Art
Upon exposure to an external wind while remaining stationary, a fan motor using, for instance, a DC motor as a fan motor rotates in reverse, thereby producing an induced electromotive voltage. When a brisk wind arises, the fan motor rotates in reverse at high speed, whereby the resultant induced electromotive voltage becomes higher. When the voltage of a power circuit becomes anomalously higher, circuit components may become defective. For this reason, there have been proposed techniques for controlling a drive circuit, by preventing an excessive increase in the voltage of the power circuit, which would otherwise be caused by the induced electromotive voltage. The technique described in Japanese Patent Laid-open No. 2001-37276A is one such technique. FIG. 8 shows a circuit described in the foregoing publication.
As shown in FIG. 8, an H bridge circuit 4 constituted by connecting transistors in the shape of the letter H comprises two pairs of output transistors; that is, a pair of power-side output transistors and a pair of ground-side output transistors. A DC motor M is connected to a node between the two output transistor pairs. A drive current is supplied to the motor M by use of the two output transistor pairs, thereby activating the motor M. A junction P of the pair of power-side output transistors is connected to a power-circuit 2. A junction of the pair of ground-side output transistors is connected to the ground. The power circuit 2 is connected to a source voltage detection circuit 3 composed by connecting in series a plurality of Zener diodes D5, D6, and D7. The source voltage detection circuit 3 monitors a supply voltage. If the supply voltage has exceeded a predetermined level as a result of reverse rotation of the motor M, the Zener diodes D5, D6, and D7 constituting the supply voltage detection circuit 3 are brought into conduction. As a result, a current flows into a capacitor C2, thereby boosting a voltage across the capacitor C2.
A PWM circuit 6 increases the duty factor of the DC motor in the direction of forward rotation by way of an error amplification circuit 5 while an increase in the voltage across the capacitor C2 is taken as a trigger. A logic circuit 7 controls activation or deactivation of the respective output transistors of the H bridge circuit 4, thereby applying a brake to reverse rotation of the DC motor. In this way, a generated voltage of the motor M is suppressed, thereby maintaining the supply voltage at a predetermined level or less.
Japanese Patent Laid-Open No. 09-182474A describes a technique for reducing a power loss, which would arise in a drive circuit of a brushless motor when a brake is applied to reverse rotation of the motor. In the drive circuit of the brushless motor of the related-art technique, a drive coil is constituted of three phases. Three sensors are provided for the respective-phases of the drive coil, thereby switching energization; The respective sensors detect a magnetic pole of an unillustrated rotor magnet. Outputs from the respective sensors are input to a matrix circuit after having been amplified by a hole amplifier. The matrix circuit outputs a timing signal to be used for switching energization of the drive coil of three phases. The signal is input to a motor drive current output section by way of a pre-driver.
In the drive circuit, when the direction of rotation of the motor is switched during the course of rotation by switching an F/R signal, an electrical reverse brake is applied to the motor until the direction of rotation of the motor is reversed. When a reverse brake is applied to the motor, a resistor Rf detects a drive current, and a comparison circuit compares the drive current with a predetermined current limit level. When a drive current in the direction of reverse rotation has exceeded the predetermined current limit level, a switching circuit is activated, thereby deactivating the power (source) output transistor or the ground (sink) output transistor of the drive current output section through control operation. After lapse of a predetermined period of time, the output transistor is again activated through control operation. When the drive current has exceeded the current limit level, the output transistor is again deactivated through control operation. In this way, individual switching elements of the drive current output section are controlled so as to avoid becoming unsaturated, by subjecting the output transistor to PWM control when a brake is applied to the motor.
In the DC motor drive circuit, a control function, such as the PWM circuit 6, is inactive when the power circuit 2 remains deactivated. Hence, an induced electromotive voltage of the motor M cannot be suppressed. If power is shut off abruptly during the course of energization of the motor M, the drive coil-produces a counter electromotive voltage for maintaining a motor current. However, according to the related-art technique, when power is shut off, suppressing the counter electromotive voltage becomes impossible.
Specifically, according to the related-art technique, a circuit cannot be protected from an overvoltage produced by the motor unless power is active.
In the drive circuit of the brushless motor, the following problems are encountered in performing PWM control in a reverse brake mode, which has been performed under the related-art technique.
Since the drive current increases at the time of application of a brake to reverse rotation as compared with normal times, a current limit level is achieved within a shorter period of time, and the output transistor remains subjected to deactivation control for a longer period of time. If the power-side output transistor or ground-side output transistor, which is subjected to activation control, is switched to a deactivated state during the period of deactivation control operation for reasons of energization switching operation, a regenerative path for the drive current is momentarily disconnected; whereby the drive current flows in reverse to a motor power line for seeking the regenerative path.
If the motor power source has no sink (absorbing) capacity, the drive current will lose a place to flow. A sharp increase momentarily arises in the motor drive voltage, thereby exceeding the withstand voltage of the drive circuit or supply circuit. A common measure against exceeding withstand voltage is interposing an electrolytic capacitor or Zener diode between the motor supply source and the ground, thereby imparting a sink capacity to the power source. However, such a measure involves a cost hike.
Use of an electrolytic capacitor entails a problem of insufficient suppression of an increase in source voltage, for the following reasons. Specifically, at the time of a reverse rotation braking operation, the period of time during which PWN activation control is to be performed is extremely short. Hence, supply of an external current is substantially not required. Conversely, since reverse flow of the regenerative current arises frequently, a charge current much larger than a discharge current flows through the electrolytic capacitor. For this reason, the supply voltage gradually increases each time the regenerative current flows.
The invention has been conceived to solve the drawback of the related-art technique and aims at providing an actuator drive circuit having an inductance which activates the actuator by causing a drive current to flow through the inductance, wherein, even when a power supply remains inactive, a circuit can be protected against an overvoltage.
The invention also aims at providing a drive circuit for a brushless motor capable of stopping reverse flow of a regenerative current to a source line and avoiding occurrence of a rise in a supply voltage without addition of an electrolytic capacitor or a Zener diode between a motor source and the ground, by generating a path for a regenerative current which would develop at the time of a braking operation.
The invention provides a drive circuit for an actuator having an inductance, wherein the actuator is driven by causing a drive current to flow into the inductance, comprising:
two pairs of output transistors, each pair including a power-side output transistor and a ground-side output transistor, the actuator being connected to nodes formed between each pair of the power-side and ground-side output transistors;
an overvoltage detection circuit for detecting whether or not voltages appearing at the nodes formed between each pair of the power-side and ground-side output transistors are higher than a supply voltage by a first predetermined level and whether or not the voltages are lower than a ground level by a second predetermined level;
wherein one of the ground-side output transistors is activated by an output from the overvoltage detection circuit when a voltage of the node connected to the one ground-side output transistor exceeds the supply voltage by the first predetermined level, and
one of the power-side output transistors is activated by an output from the overvoltage detection circuit when a voltage of the node connected to the one power-side output transistor becomes lower than the ground level by the second predetermined level.
Here, the overvoltage detection circuit includes a power-side detection transistor connected to a supply line and a ground-side detection transistor connected to a ground line; and
a node formed between the power-side and ground-side detection transistors is connected to one end of the inductance of the actuator.
The actuator having the inductance according to the invention corresponds typically to any one of a brushless motor, a DC motor, a voice coil motor, and an electromagnetic actuator.
In the brushless motor drive circuit of the invention, every time a regenerative current flows through a supply line in reverse, a voltage appearing at one of motor drive coils of two arbitrary phases has inevitably jutted above a supply voltage and a voltage appearing at the other motor drive coils has also inevitably jutted below a ground level. Attention is given to this phenomenon, and the phenomenon is detected by an overvoltage detection circuit. A ground-side output transistor connected to a drive coil of phase whose voltage has jutted above the supply voltage or a power-side output transistor connected to a drive coil of phase whose voltage has become below a ground level or has jutted below the ground level by a predetermined level is forcefully activated. As a result, a regenerative current path is formed within the motor drive circuit, whereby reverse flow of an electric current to the supply line is stopped, thus inhibiting an increase in the supply voltage.
In this respect, the invention is characterized by a drive circuit for a brushless motor wherein the brushless motor is rotated by causing motor a drive current to flow into motor drive coils, comprising:
a plural pairs of output transistors, each pair including a power-side output transistor and a ground-side output transistor, different motor drive coils being connected to nodes formed between each pair of the power-side and ground-side output transistors;
an overvoltage detection circuit for detecting whether or not voltages appearing at the nodes formed between each pair of the power-side and ground-side output transistors are higher than a supply voltage by a first predetermined level and whether or not the voltages are lower than a ground level by a second predetermined level;
wherein one of the ground-side output transistors is activated by an output from the overvoltage detection circuit when a voltage of the node connected to the one ground-side output transistor exceeds the supply voltage by the first predetermined level, and
one of the power-side output transistors is activated by an output from the overvoltage detection circuit when a voltage of the node connected to one power-side output transistor becomes lower than the ground level by the second predetermined level.
Further, the invention is characterized by a drive circuit for a brushless motor having a sensor for detecting a rotary position of a rotor wherein the motor is rotationally driven by causing a motor drive current to flow into motor drive coils in accordance with a position signal detected by the sensor, and a an braking operation is performed by causing a motor drive current to flow in a direction in which the motor is to be rotated in reverse, the drive circuit comprising:
a drive current detection unit for detecting the motor drive current;
a comparator for comparing a predetermined reference current value with the motor drive current value detected by the drive current detection unit;
a current control switching section for activating or deactivating a motor drive current output section in accordance with a result of comparison performed by the comparator; and
an overvoltage detection circuit for detecting whether or not a voltage appearing at a terminal of the motor drive coils is higher than a supply voltage by a first predetermined level and whether or not the voltage is lower than a ground level by a second predetermined level;
wherein a regenerative current flow path is formed in the motor drive current output section by an output from the overvoltage detection circuit.
According to the invention, the motor drive coil is constituted of three phases; and the overvoltage detection circuit can be constructed so as to detect that a voltage appearing at a terminal of one motor drive coil from among two motor drive coils of two arbitrary phases is higher than the supply voltage by the first predetermined level and that a voltage appearing at the remaining motor drive coil is lower than the ground level by the second predetermined level.